Electrooxidation of alcohols catalyzed by amino alcohol ligated ruthenium complexes

Article abstract of DOI:10.1021/ja4055564

Ruthenium transfer hydrogenation catalysts physisorbed onto edge-plane graphite electrodes are active electrocatalysts for the oxidation of alcohols. Electrooxidation of CH₃OH (1.23 M) in a buffered aqueous solution at pH 11.5 with [(η⁶-p-cymene)(η²-N,O-(1R,2S)-cis-1- amino-2-indanol)]Ru^IICl (2) on edge-plane graphite exhibits an onset current at 560 mV vs NHE. Koutecky-Levich analysis at 750 mV reveals a four-electron oxidation of methanol with a rate of 1.35 M^-1 s ^-1. Mechanistic investigations by ¹H NMR, cyclic voltammetry, and desorption electrospray ionization mass spectrometry indicate that the electroxidation of methanol to generate formate is mediated by surface-supported Ru-oxo complexes.

Full text of DOI:10.1021/ja4055564

Article
pubs.acs.org/JACS
Electrooxidation of Alcohols Catalyzed by Amino Alcohol Ligated
Ruthenium Complexes
Kristen R. Brownell, Charles C. L. McCrory, Christopher E. D. Chidsey, Richard H. Perry,
Richard N. Zare, and Robert M. Waymouth*
Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States
*
S Supporting Information
ABSTRACT: Ruthenium transfer hydrogenation catalysts physisorbed onto
edge-plane graphite electrodes are active electrocatalysts for the oxidation of
alcohols. Electrooxidation of CH OH (1.23 M) in a buffered aqueous solution
3
6
2
at pH 11.5 with [(η -p-cymene)(η -N,O-(1R,2S)-cis-1-amino-2-indanol)]-
Ru Cl (2) on edge-plane graphite exhibits an onset current at 560 mV vs
II
NHE. Koutecky−Levich analysis at 750 mV reveals a four-electron oxidation
−1
−1
1
of methanol with a rate of 1.35 M s . Mechanistic investigations by H
NMR, cyclic voltammetry, and desorption electrospray ionization mass
spectrometry indicate that the electroxidation of methanol to generate formate
is mediated by surface-supported Ru−oxo complexes.
INTRODUCTION
Scheme 1. Transfer Hydrogenation of Ketones by Alcohols
■
Alcohols are attractive chemical fuels for fuel cells due to their
high energy densities, established production and distribution
infrastructure, and ability to be derived from renewable
1
−3
resources. However, even the most highly optimized alcohol
electrooxidation catalysts suffer from kinetic limitations that
2
−11
require high overpotentials to achieve reasonable rates.
Polypyridyl ruthenium−oxo complexes are among the most
highly developed class of alcohol electrooxidation com-
1
2−14
13,15−19
plexes
but require relatively large overpotentials.2
0,21
We targeted transfer hydrogenation (TH) catalysts
as a
potentially attractive class of candidates for alcohol electro-
oxidation catalysts, since catalytic transfer hydrogenation
involves the reversible oxidation of alcohols with ketones as
terminal oxidants. In a typical transfer hydrogenation, an
alcohol serves as a hydrogen donor to reduce a ketone (Scheme
1). The chemical reversibility of TH reactions suggests that the
catalysts mediating these transformations are operating close to
their reversible redox potentials and thus are promising
candidates for reversible alcohol electrooxidation catalysts.
Transfer hydrogenation of ketones by alcohols with cymene Ru
2
1,22
complexes was reported by Noyori and Ikariya.
studies
Mechanistic
2
1−26
indicate that the concerted dehydrogenation of
alcohols by the Ru amide A generates a ketone and the Ru−H
B, which in turn can reduce a ketone to regenerate A (Scheme
1
).
Due to their promising reactivity in transfer hydrogenation,
Figure 1. Cymene Ru complexes.
we investigated complexes 1 and 2 as molecular electrocatalysts
for alcohol oxidation (Figure 1). Complex 2, derived from
Herein we show that the complexes 1a and 2 are active for
the electrocatalytic oxidation of alcohols when supported on
[
Ru(p-cymene)Cl2]2 and (1R,2S)-cis-1-amino-2-indanol, is
27,28
more active than the Ru−Cl complex 1
and exhibits
−1
turnover frequencies of up to 540 h for transfer hydro-
genations of ketones with 2-propanol at room temperature with
Received: June 17, 2013
Published: September 17, 2013
28
27,28
2.5 mol % KOH, nearly 60 times faster than complex 1.
©
2013 American Chemical Society
14299
dx.doi.org/10.1021/ja4055564 | J. Am. Chem. Soc. 2013, 135, 14299−14305

Journal of the American Chemical Society
Article
edge-plane graphite (EPG) electrodes in basic aqueous
solution. When physisorbed onto EPG electrodes, complex 2
exhibits rapid rates for methanol oxidation by four electrons at
slopes of the first several points of the acetophenone
conversion vs time plots.
The catalytic transfer hydrogenation of acetophenone with
methanol in the presence of complexes 1a, 1, and 2 affords 1-
phenylethanol and methyl formate in a ratio of 2:1, indicating
that methanol provides two reducing equivalents. Methyl
formate is observed as the only product of methanol
7
1
50 mV vs NHE and pH 11.5. The structurally similar complex
also exhibits rapid rates for 2-propanol electrooxidation, albeit
slower than those for 2. Proton NMR, cyclic voltammetry, and
desorption electrospray ionization mass spectrometry studies
on the electrode surface before and after catalysis indicate that
the four-electron-oxidation product of methanol is formate.
1
dehydrogenation via H NMR and GC-MS. The oxidation of
31−33
methanol to methyl formate
by Ru(II) complexes has
been observed previously and likely proceeds by the initial
dehydrogenation of methanol to formaldehyde, followed by
condensation with methanol to the acetal and subsequent
RESULTS AND DISCUSSION
■
Synthesis. Complexes 1 and 1a,b were prepared as
2
2
31−36
previously described. Attempts to prepare the Ru−Cl
dehydrogenation.
2
8,29
22
complex 2
analogously to 1 were unsuccessful, resulting
These results reveal that the initial rates for the TH of
acetophenone were faster for 2-propanol than for methanol and
that for both alcohols the catalyst derived from the Ru−Cl
complex 2, derived from the amino alcohol ligand, was more
active than that derived from the diamide 1 or 1a. The latter
in a mixture of the ruthenium chloride and hydride species.
Instead, 2 was prepared by the dropwise addition of a THF
solution of [(p-cymene)RuCl] to a THF solution of the
2
sodium salt of (1R,2S)-cis-1-amino-2-indanol at −78 °C.
Recrystallization from diethyl ether/pentane at reduced
temperature afforded 2 in analytically pure form.
28
30
result is consistent with the reports of Wills and Blacker
that catalysts derived from amino alcohol ligands are more
active for transfer hydrogenation than those from diamide
ligands.
Transfer Hydrogenation. Catalytic transfer hydrogena-
tions of ketones typically employ 2-propanol or formate as a
hydrogen donor. While methanol has been investigated
Electrocatalytic Alcohol Oxidation. Electrocatalytic
oxidation of methanol and 2-propanol were investigated by
rotating disk electrode (RDE) voltammetry with samples of the
Ru−Cl complex 2 and the Ru−amide complex 1a deposited
onto edge-plane graphite (EPG) electrodes (Table 2, Figure 2).
Electrochemical measurements were recorded with a potentio-
stat using an MSR rotator, an auxiliary Pt-wire electrode, and a
Ag/AgCl/KCl (sat) reference electrode in aqueous solutions of
2
2
previously as a hydrogen donor for the transfer hydro-
30
genation of ketones, it is less effective than 2-propanol. To
compare the reactivities of the two alcohols with complexes 1
and 2, the catalytic transfer hydrogenation of acetophenone by
both methanol and 2-propanol was performed to compare the
relative reactivity of the diamide 1a and the Ru chlorides 1 and
2
(Scheme 2 and Table 1). While 1a does not require a base for
0
.01 M phosphate and 0.1 M sodium perchlorate. The pH was
Scheme 2. Transfer Hydrogenation of Acetophenone (1.22
M) with (a) 2-Propanol and (b) Methanol Employing 1a, 1,
and 2
adjusted by addition of 1.0 M solutions of either NaOH or
HClO4.
We investigated the electrocatalytic oxidation of both
methanol and 2-propanol with the amino alkoxide Ru−Cl
complex 2. The cyclic voltammogram (CV) of 2 physisorbed
onto EPG in a 0.1 M sodium perchlorate solution at pH 5.4
under an N atmosphere exhibits a quasi-reversible oxidation at
2
E1/2 = 540 mV vs NHE (ΔE = 460 mV) under a nitrogen
p‑p
II/III
atmosphere, which is attributed to the Ru
couple (Figure
1
2,37,38
S4a, Supporting Information).
Plots of both the anodic
and cathodic peak currents as a function of scan rate are linear,
confirming that the electroactive species is surface adsorbed
(Figure S4b−d, Supporting Information). A surface coverage of
Table 1. Initial Turnover Frequencies for Transfer
Hydrogenation Catalyzed by 1, 1a, and 2
a
.14 × 10− mol/cm of electroactive Ru on the EPG electrode
10
2
2
catalyst
alcohol
concn of sodium alkoxide (M)
TOF (h−1)
II/III
i
was estimated by integration of the area of the Ru
redox
1
1
2
1
1
2
a
methanol
methanol
methanol
2-propanol
2-propanol
2-propanol
0.17 (methoxide)
6.5
8.5
21
peak at pH 5.4. CVs obtained at pH values from pH 2.28 to
1.75 revealed that the anodic peak shifts to more negative
0.17 (methoxide)
1
0.17 (methoxide)
potentials with increasing pH at −68 mV per pH unit (Figure
S5, Supporting Information).
a
0.035 (isopropoxide)
0.035 (isopropoxide)
0.035 (isopropoxide)
18
26
Electrocatalytic oxidation of methanol catalyzed by 2 was
investigated by RDEV in 0.1 M sodium perchlorate aqueous
solution with 0.01 M phosphate buffer. When the Ru−Cl 2 is
adsorbed onto an edge-plane graphite electrode in 1.23 M
methanol solution at pH 11.5, an electrocatalytic current is
observed with an onset potential of 560 mV vs NHE (Figure
113
a
Reactions performed under inert conditions at 40 °C in CD CN with
.22 M acetophenone, 3.66 M 2-propanol or 9.27 M methanol, and 8.9
3
1
mM catalyst with p-xylene as an internal standard. TOF = (mmol of
i
acetophenone consumed)/((mmol catalyst) time), measured at 0.25
h.
2
3
a). Analysis of the current as a function of rotation rate (400−
39
000 rpm, Figure S4) by Koutecky−Levich analysis provides
estimates of both the kinetic rate constant and the number of
electrons involved in the electrocatalytic process. Plots of
i(E) , the inverse of the current measured at a constant
potential E = 750 mV vs NHE, as a function of ω
iK(E) as the intercept (Figure 2b). This kinetic current was
activity, reactions with methanol and 2-propanol were carried
out in the presence of the alkoxide to provide comparable
experimental conditions. The reactions were monitored by H
−1
1
−1/2
yield
−1
39
NMR, and initial turnover frequencies were calculated from the
1
4300
dx.doi.org/10.1021/ja4055564 | J. Am. Chem. Soc. 2013, 135, 14299−14305

Journal of the American Chemical Society
Article
Table 2. Electrocatalytic Alcohol Oxidation by Complexes 2 and 1a on EPG Electrodes
coverage 10− mol/cm2
10
Epa, mV (pH)
alcohol
pH
n
iK, μA
k, M−1 s−1
complex
a
2
2
2.1
2.1
2.2
770 (5.4)
MeOH
iPrOH
iPrOH
iPrOH
11.5
11.5
11.5
4
2
2
2
14.9
1.35
13.9
2.01
10.2
a
84.0
b
1a
a
1040 (5.96)
19.5
c
1
2.2
5.96
38.9
a
b
c
E = 750 mV vs NHE. E = 650 mV vs NHE. E = 1.4 V vs NHE.
constant of k = 13.9 M− s−1, considerably faster than methanol
electrooxidation under similar conditions (Table 1 and Figure
S12 (Supporting Information)).
1
Electrooxidation of 2-propanol with diamide 1a displayed an
onset of electrocatalytic current at 603 mV vs NHE, pH 11.5
(
Figure S10, Supporting Information). The rate of 2-propanol
electrooxidation by the diamide 1a was considerably slower
−
1
than that by the amino alcohol complex 2 (k = 2.01 vs 13.9 M
−1
s , respectively; Table 2). These relative rates observed in the
electrocatalytic alcohol oxidation (Table 2) mirror the trends
observed in the catalytic transfer hydrogenation reactions
28
(
Table 1): complex 2 exhibits faster rates than complex 1a,
and the transfer hydrogenation of acetophenone with 2-
22,40
propanol is considerably faster than that with methanol.
These data provide strong if indirect support that the bidentate
ligands remain coordinated to Ru during electrocatalysis. This
conclusion is also supported by control experiments, which
showed that [Ru(p-cymene)Cl ] , lacking the bidentate ligands,
2
2
is not an effective electrocatalyst for 2-propanol oxidation under
similar conditions (Figures S15−S17, Supporting Information).
The results of the electrocatalytic experiments show that the
molecular Ru cymene complexes 1a and 2 mediate the
electrocatalytic oxidation of aliphatic alcohols at fast rates,
albeit at relatively high potentials. The rate constants for
methanol and 2-propanol oxidation (650−750 mV vs NHE, pH
1
1.5) correspond to turnover frequencies of approximately 1
−1
i
−1
MeOH s and 8 PrOH s . These rates are attractive for fuel
cell applications and are comparable to those reported for
MeOH electrooxidation with Pd nanoparticle catalysts (1.1
−1
41
Figure 2. (a) Rotating disk electrode voltammogram of 2 (blue)
physisorbed onto EPG in a 1.23 M MeOH aqueous solution at 3000
rpm. The onset of electrocatalytic current (blue) appears at 560 mV
over background current (black) of a polished EPG surface in the
mA/μg Pd, ∼0.3 MeOH s ) but lower than that reported for
4
−1 −1 42,43
supported Ni catalysts (k = 7.4 × 10 M s ).
For
molecular Ru electrocatalysts, rate constants for alcohol
3
−1 −1
electrooxidation range from k = 0.03 to 6.6 × 10 M s ,
depending on the alcohol and the coordination geometry at
Ru.
same solution. Conditions: supporting electrolyte 0.1 M NaClO , 0.01
4
M phosphate buffer, pH 11.5, scan rate 100 mV/s, N atmosphere. (b)
2
13,15−19,38,44−50
For example, the Ru−-oxo complex
Koutecky−Levich plot of the inverse of the plateau current measured
IV
2+
−1/2
at 750 mV vs NHE as a function of (rotation rate) . The colored
lines give the calculated responses for a diffusion-controlled oxidation
of methanol by two (red), four (green), and six (blue) electrons.
[Ru (trpy)(phen)O] when adsorbed onto a carbon paste
electrode electrooxidizes methanol at 0.030 M−
1
s
−1
at 0.285 V
1
5
vs SSCE at pH 13, an overpotential of 1.1 V. The related
IV
[
Ru (bpy) (py)O] complex oxidizes methanol with a rate
2
−2
−4
−1 −1
determined to be i (E) = 14.9 μA (85.1 μA cm ). The plot of
constant of 3.5 × 10
NHE). Faster rates were reported for methanol electro-
M
s
at 0.99 V vs SCE (1.23 V vs
K
51
the inverse currents is parallel to the inverse theoretical Levich
3
9
V
current calculated with n = 4 (Figure 2b), indicative of a four-
electron oxidation of methanol to either formate or carbon
monoxide (eq 1). This analysis yields a rate constant of k = 1.35
oxidation with [Ru [bis(2-pyridylmethyl)ethylenediamine]Cl-
(O)] (k = 6.6 × 10 M
2
−1 −1
s
at 1.3 V vs SCE; 1.54 V vs
16
IV
2+
NHE) or [Ru (dcbpy) (O) ] (dcbpy = 6,6′-dichloro-2,2′-
2
s
2
bipyridine) (k = 3.3 M−
1
−1
at 1.39 V vs NHE).
17
−
−
−
CH OH + 5OH → HCO + 4e + 4H O
(1)
3
2
2
The onset of electrocatalytic current for methanol oxidation
by 2 reported here occurs at 560 mV vs NHE (pH 11.5, Figure
2a), over 1000 mV positive of the thermodynamic potential of
M− s−1 and a turnover frequency of approximately one
methanol/s for the electrocatalytic oxidation of methanol by
the amino alcohol complex 2 at 750 mV vs NHE.
Electroxidation of 2-propanol by 2 exhibited an onset
potential of 500 mV vs NHE at pH 11.5. Koutecky−Levich
analysis at 650 mV vs NHE is consistent with a two-electron
oxidation at a rate of 15.8 electrons/s, corresponding to a rate
1
52
methanol oxidation to formate. These values are more
positive than that reported for Pd nanoparticle catalysts
(−0.150 V vs Ag/AgCl/saturated KCl, 50 mV vs NHE; pH
41
14.4) but comparable to those of other supported or solution-
1
3,15−19,53−55
phase molecular electrocatalysts based on Ru,
4
2,43
9,10
Ni,
or Rh.
1
4301
dx.doi.org/10.1021/ja4055564 | J. Am. Chem. Soc. 2013, 135, 14299−14305

Journal of the American Chemical Society
Article
Proposed Electrocatalytic Mechanism. Transfer hydro-
genation catalysts such as complexes 1 and 2 were targeted as
promising electrooxidation catalysts, as they mediate the
reversible oxidation of alcohols and reduction of ketones by
Ru−H and Ru−amide intermediates. The results of our
electrocatalytic experiments in water suggest that the electro-
catalytic oxidation of methanol and 2-propanol proceed by an
conditions would be expected to generate Ru−oxo inter-
mediates (which may or may not retain the arene ligand).
Related arene RuO intermediates have been proposed to
6
1,62
mediate the oxidation of alcohols.
The voltammetry of complexes 1a and 2 is consistent with
this hypothesis, as the anodic peaks in the CVs of 1a and 2
physisorbed on EPG are dependent upon pH and exhibit
Nernstian responses of ca. −59 mV per pH unit, indicative of a
proton-coupled electron transfer. Such behavior is typical of
−1
alternative mechanism. The electrocatalytic rate (ca. 1 s ) for
methanol oxidation by 2 is much faster than the chemical rate
−1
of transfer hydrogenation (ca. 21 h ) at room temperature. In
R u − O H / R u − O H / R u  O r e d o x p r o -
2
1
2,18,38,44−47,49,51,63,64
addition, the potentials at which electroxidation occurs (650−
cesses.
Moreover, the overpotentials calcu-
7
50 mV vs NHE) are more positive than those expected for
lated (approximately 1 V) are consistent with those observed
for other electrocatalytic alcohol oxidations proposed to occur
56,57
Ru−H intermediates.
The irreversible oxidation of the Ru−
1
3,63
H complex 1b measured in CH CN occurs at −243 mV vs
3
by RuO intermediates.
+
/0
Cp Fe (Figure S18, Supporting Information). While transfer
2
To provide evidence for this proposed mechanism, we
employed desorption electrospray ionization mass spectrome-
hydrogenation is known to proceed in water with complexes
5
8
65−67
1,
water reacts rapidly with the Ru−amide 1a to generate a
try (DESI)
Figure 3). The development of ambient ionization methods in
the past decade has provided new strategies to detect reactive
to examine species on the electrode surfaces
58
Ru−hydroxo complex (Figure S19, Supporting Information).
These data imply that the electrocatalytic mechanism is not
(
65
mediated by ruthenium hydrides but more likely by Ru−
intermediates in solution using mass spectrometry (MS).
1
2,19,44−46,59,60
hydroxo or Ru−oxo complexes.
DESI, a recently developed ionization method, requires little
Shown in Scheme 3 is a proposed catalytic mechanism where
sample preparation and can be carried out in an open
aquation of the Ru cymene precursors generates Ru−OH
65−67
environment.
In a DESI experiment, charged droplets in
a stream of gas are sprayed on a surface containing a deposited
analyte and extract analyte molecules from the surface into
secondary microdroplets from which gas-phase ions are
Scheme 3. Proposed Mechanism for Electrocatalytic Alcohol
Oxidation Involving Ru(IV)−Oxo Intermediates
a
68
formed. By addition of reagents to the DESI spray, one can
perform reactions with compounds adsorbed onto surfaces,
intercept short-lived intermediates, and monitor reaction
68
products in real time. Recently, nano-DESI was employed
69
to study reactions on electrode surfaces, indicating the power
of ambient mass spectrometric techniques in probing electro-
chemical processes. Here we apply DESI to characterize species
after RDE electrocatalysis. These results demonstrate that DESI
is a powerful technique to study electrode surfaces, as the
measurement can be performed rapidly (<5 min including any
instrument preparation time) and can help identify reactive
intermediates, including those present in low abundance.
An initial DESI experiment was carried out by depositing
complex 2 on edge-plane graphite from an acetone solution and
subsequently analyzing the surface species by spraying
methanol onto the electrode. The electrode was affixed to a
glass slide, and methanol (degassed via sparging with argon)
was sprayed onto the surface by a DESI source (0 kV spray
a
L = cymene, solvent, or EPG surface.
intermediates (or Ru−OH intermediates, depending on the
pH), which under relatively high potentials and basic
2
−2
Figure 3. Schematic of DESI experiment to detect species present after electrocatalysis with 2 (3 × 2 μL of a 10 M solution in acetone deposited
−1
−1
on edge-plane graphite) using CH OH infused at 5 μL min (N flow rate 0.6 L min ).
3
2
1
4302
dx.doi.org/10.1021/ja4055564 | J. Am. Chem. Soc. 2013, 135, 14299−14305

Journal of the American Chemical Society
Article
3
1
voltage). The DESI spectrum of complex 2 supported on EPG
previously in a DESI experiment where a methanol solution
of the amino alcohol ligand was sprayed onto a surface
(
Figure 4a) shows several species, including the Ru−Cl cation
70
containing the p-cymene Ru dimer [(p-cymene)RuCl ] .
2
2
To analyze the Ru complexes after electrocatalysis, a sample
of complex 2 supported on EPG was immersed in an aqueous
solution containing 1.23 M MeOH, 0.1 M sodium perchlorate,
and 0.01 M phosphate buffer at pH 11.5 and subjected to a
series of rotating disk electrode voltammograms from 200 to
1
200 mV vs NHE, exactly as done for the electrocatalytic
kinetics analysis. The electrode was removed from the solution,
allowed to dry in air, and analyzed by DESI by spraying
degassed methanol onto the electrode. The DESI spectrum
shown in Figure 4b reveals that the major ion observed is that
+
at m/z 384.0907, corresponding to the cation 2a . The ion at
m/z 420.0667 corresponding to 2 decreased significantly in
abundance (Figure 4b), implicating that most of the Ru−Cl
precursor is consumed during electrocatalysis. Notably, the
peaks at m/z 689.04300 and 838.1272 are no longer observed,
suggesting that the chlorides are displaced during the course of
+
the experiment. The observation that 2a is observed as the
major species following electrooxidation (Figure 4b) implies
that a significant fraction of the Ru complexes on the surface
retain both the cymene ligand and the amino alcohol ligand
under an applied potential. This experiment attests to the
stability of the complex 2a under the electrocatalytic conditions,
though it does not allow us to infer that 2a is the active catalyst.
Additionally, several minor species detected in the DESI
mass spectrum could be assigned unambiguously (Figures 4
and 5; errors were <3.5 ppm and at least three of the most
intense peaks of the isotope distribution were observed in the
71
mass spectrum). We assign the ion at m/z 424.0826 to the
II
+
Ru −OH species 2dNa (0.71 ppm) (Figure 4c) and that at
IV
+
m/z 482.0881 to the Ru −methoxy formate species 2e (1.0
ppm) (Figure 4d). Also identified, albeit in very low abundance,
were species assigned as the Ru −formic acid adduct 2f at m/
z 430.09556 (2.6 ppm) and Ru −hydroxy formate species 2g
II
+
IV
+
at m/z 446.0900 (3.4 ppm) (Figures S20 and S21, Supporting
Information). This information provides indirect evidence for
the formation of these Ru species on the EPG electrode surface
under an applied potential. Although these ions were observed
on the electrode surface following the electrochemistry
experiments, we cannot completely exclude the possibility
that they were formed in the DESI experiment in the secondary
microdroplets. However, significantly, this technique enables
one to obtain information not possible to collect by any other
means.
Figure 4. DESI mass spectra of (a) complex 2 supported on an EPG
electrode and (b) 2/EPG following electrocatalysis of methanol. The
peak at m/z 420.0677 decreases in abundance and m/z 384.0911
becomes the predominant species. DESI mass spectra showing (c) the
peak at m/z 424.0829 and (d) the peak at m/z 482.0886.
2+
(m/z 420.0667), species 2a at (m/z 384.0907), the cationic
+
+
dimer [(p-cymene)RuCl-Ru(Cl) (p-cymene)] (m/z
2
+
576.9340), and the dimer [2+2] (m/z 838.1272). The ions
observed at m/z 384.0907 and 420.0667 provide evidence that
the Ru−Cl complex 2 remains intact when supported on edge-
plane graphite electrodes. These species were observed
Catalytic transfer hydrogenation experiments reveal that the
chemical oxidation product of methanol by 2 is methyl
31
formate. Moreover, our DESI studies on the EPG electrodes
Figure 5. Ions observed on the edge-plane graphite electrode following electrocatalysis.
1
4303
dx.doi.org/10.1021/ja4055564 | J. Am. Chem. Soc. 2013, 135, 14299−14305

Journal of the American Chemical Society
Article
have identified species containing formic acid or formate
subsequent to the electrocatalytic process that were not present
in the absence of an applied potential (Figures 4 and 5). No
masses were observed corresponding to proposed Ru−CO or
(6) Liu, H.; Song, C.; Zhang, L.; Zhang, J.; Wang, H.; Wilkinson, D.
P. J. Power Sources 2006, 155, 95−110.
(
(
7) Hori, Y. Mod. Aspects Electrochem. 2008, 42, 89−189.
8) Fachini, E. R.; Diaz-Ayala, R.; Casado-Rivera, E.; File, S.; Cabrera,
C. R. Langmuir 2003, 19, 8986−8993.
9) Annen, S. P.; Bambagioni, V.; Bevilacqua, M.; Filippi, J.;
Marchionni, A.; Oberhauser, W.; Schonberg, H.; Vizza, F.; Bianchini,
C.; Grutzmacher, H. Angew. Chem., Int. Ed. 2010, 49, 7229−7233.
10) Bevilacqua, M.; Bianchini, C.; Marchionni, A.; Filippi, J.;
Ru−CO species. This evidence indicates that the Ru cymene
2
(
complexes 1 and 2 serve as active precursors for the
electrocatalytic oxidation of methanol to formate.
̈
̈
(
CONCLUSIONS
■
Lavacchi, A.; Miller, H.; Oberhauser, W.; Vizza, F.; Granozzi, G.;
Artiglia, L.; Annen, S. P.; Krumeich, F.; Grutzmacher, H. Energy
Environ. Sci. 2012, 5, 8608−8620.
Ruthenium arene transfer hydrogenation complexes are active
electrocatalytic precursors for alcohol oxidation when sup-
ported on edge-plane graphite electrodes in basic aqueous
solution. Amino alcohol chloride 2 exhibits a rate of
approximately 1 turnover/s for methanol electrooxidation by
four electrons at 750 mV vs NHE with an onset of
electrocatalytic current at 560 mV vs NHE. The use of DESI-
(11) Elouarzaki, K.; Le Goff, A.; Holzinger, M.; Thery, J.; Cosnier, S.
J. Am. Chem. Soc. 2012, 134, 14078−14085.
12) Meyer, T. J.; Huynh, M. H. V. Inorg. Chem. 2003, 42, 8140−
160.
13) Cheung, K.-C.; Wong, W.-L.; Ma, D.-L.; Lai, T.-S.; Wong, K.-Y.
Coord. Chem. Rev. 2007, 251, 2367−2385.
14) Hu, Z.; Du, H.; Leung, C.-F.; Liang, H.; Lau, T.-C. Ind. Eng.
Chem. Res. 2011, 50, 12288−12292.
15) Kutner, W.; Meyer, T. J.; Murray, R. W. J. Electroanal. Chem.
985, 195, 375−394.
(
8
(
1
MS experiments on electrode surfaces and H NMR experi-
(
ments imply that 2 is capable of oxidizing methanol by four
electrons to formate species. The Nernstian dependence on the
anodic peak potential in the CV of 2 on EPG coupled with the
slow rates of TH for 2 and the relatively high potentials
required for observation of electrocatalysis (ca. 1.2 V above the
thermodynamic oxidation potential of methanol to formate)
lead us to conclude that the electrocatalytic oxidation of
methanol to formate by 2 on edge-plane graphite electrodes in
water is mediated by RuO species rather than Ru−H species.
(
1
(16) Li, C. K.; Tang, W. T.; Che, C. M.; Wong, K. Y.; Wang, R. J.;
Mak, T. C. W. J. Chem. Soc., Dalton Trans. 1991, 1909−1914.
(17) Lai, Y. K.; Wong, K. Y. Electrochim. Acta 1993, 38, 1015−1021.
(
18) Gerli, A.; Reedijk, J.; Lakin, M. T.; Spek, A. L. Inorg. Chem.
995, 34, 1836−1843.
19) Hornstein, B. J.; Dattelbaum, D. M.; Schoonover, J. R.; Meyer,
T. J. Inorg. Chem. 2007, 46, 8139−8145.
20) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226−236.
1
(
(
ASSOCIATED CONTENT
Supporting Information
■
(21) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300−1308.
(22) Haack, K. J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R.
Angew. Chem., Int. Ed. 1997, 36, 285−288.
*
S
Text and figures giving experimental methods, NMR spectra,
cyclic voltammetry, mass spectra, synthesis details, and
characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
(23) Hashiguchi, S.; Fujii, A.; Haack, K. J.; Matsumura, K.; Ikariya, T.;
Noyori, R. Angew. Chem., Int. Ed. 1997, 36, 288−290.
(24) Ikariya, T.; Murata, K.; Noyori, R. Org. Biomol. Chem. 2006, 4,
3
93−406.
25) Matsumura, K.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am.
Chem. Soc. 1997, 119, 8738−8739.
26) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem. 2001,
66, 7931−7944.
27) Takehara, J.; Hashiguchi, S.; Fujii, A.; Inoue, S.-i.; Ikariya, T.;
Noyori, R. Chem. Commun. 1996, 233−234.
28) Palmer, M.; Walsgrove, T.; Wills, M. J. Org. Chem. 1997, 62,
(
AUTHOR INFORMATION
Corresponding Author
waymouth@stanford.edu.
■
(
(
Notes
The authors declare no competing financial interest.
(
5
226−5228.
ACKNOWLEDGMENTS
■
(29) Kenny, J. A.; Wills, M.; Versluis, K.; Heck, A. J. R.; Walsgrove, T.
We acknowledge the NSF (CHE-1213403) and the Global
Climate and Energy Program at Stanford. R.H.P. and R.N.Z.
thank the Air Force Office of Scientific Research for support of
this project under AFOSR FA9550-10-1-0235. K.B. thanks the
Center for Molecular Analysis and Design (CMAD) at Stanford
for a graduate fellowship; C.C.L.M. thanks the Ford
Foundation for a Dissertation Fellowship. We also thank
Pavel Aronov and Allis Chien (Stanford University Mass
Spectrometry) for providing mass spectrometry instrumenta-
tion.
Chem. Commun. 2000, 99−100.
(30) Blacker, J., Martin, J. Asymmetric Catalysis on Industrial Scale;
Wiley: New York, 2004.
(
31) Perry, R. H.; Brownell, K. R.; Chingin, K.; Cahill, T. J.;
Waymouth, R. M.; Zare, R. N. Proc. Natl. Acad. Sci. U.S.A. 2012, 109,
246−2250.
32) Smith, T. A.; Aplin, R. P.; Maitlis, P. M. J. Organomet. Chem.
985, 291, C13−C14.
33) Yang, L. C.; Ishida, T.; Yamakawa, T.; Shinoda, S. J. Mol. Catal.
A: Chem. 1996, 108, 87−93.
34) Pearson, D. M.; Waymouth, R. M. Organometallics 2009, 28,
896−3900.
35) Nishimura, T.; Kakiuchi, N.; Onoue, T.; Ohe, K.; Uemura, S. J.
2
(
1
(
(
3
(
REFERENCES
■
(
1) Olah, G. A. Angew. Chem., Int. Ed. Engl. 2005, 44, 2636−2639.
2) Rabis, A.; Rodriguez, P.; Schmidt, T. J. ACS Catal. 2012, 2, 864−
Chem. Soc., Perkin Trans. 1 2000, 1915−1918.
(
(36) Nielsen, M.; Alberico, E.; Baumann, W.; Drexler, H. J.; Junge,
H.; Gladiali, S.; Beller, M. Nature 2013, 495, 85−89.
(37) Thompson, M. S.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104,
4106−4115.
890.
(
3) Krewer, U.; Vidakovic-Koch, T.; Rihko-Struckmann, L.
ChemPhysChem 2011, 12, 2518−2544.
(
(
4) Bianchini, C.; Shen, P. K. Chem. Rev. 2009, 109, 4183−4206.
(38) Llobet, A. Inorg. Chim. Acta 1994, 221, 125−131.
(39) Bard, A. J., Faulkner, L. R. Electrochemical Methods:
Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001.
(40) Brownell, K. B.; Waymouth, R. M. Manuscript in preparation.
5) Gurau, B.; Viswanathan, R.; Liu, R.; Lafrenz, T. J.; Ley, K. L.;
Smotkin, E. S.; Reddington, E.; Sapienza, A.; Chan, B. C.; Mallouk, T.
E.; Sarangapani, S. J. Phys. Chem. B 1998, 102, 9997−10003.
1
4304
dx.doi.org/10.1021/ja4055564 | J. Am. Chem. Soc. 2013, 135, 14299−14305

Journal of the American Chemical Society
Article
(
41) Bambagioni, V.; Bianchini, C.; Marchionni, A.; Filippi, J.; Vizza,
F.; Teddy, J.; Serp, P.; Zhiani, M. J. Power Sources 2009, 190, 241−251.
42) Raoof, J.-B.; Ojani, R.; Hosseini, S. R. J. Power Sources 2011, 196,
(
1
(
(
855−1863.
43) Zheng, L.; Song, J.-f. J. Solid State Electrochem. 2010, 14, 43−50.
44) Chan, S. L.-F.; Kan, Y.-H.; Yip, K.-L.; Huang, J.-S.; Che, C.-M.
Coord. Chem. Rev. 2011, 255, 899−919.
45) Che, C.-M.; Cheng, K.-W.; Chan, M. C. W.; Lau, T.-C.; Mak,
C.-K. J. Org. Chem. 2000, 65, 7996−8000.
46) Che, C.-M.; Ho, C.; Lau, T.-C. J. Chem. Soc., Dalton Trans. 1991,
901−1907.
47) Che, C.-M.; Yip, W.-P.; Yu, W.-Y. Chem. Asian J. 2006, 1, 453−
58.
48) Laurent, F.; Plantalech, E.; Donnadieu, B.; Jimenez, A.;
Hernandez, F.; Martinez-Ripoll, M.; Biner, M.; Llobet, A. Polyhedron
999, 18, 3321−3331.
49) Sens, C.; Rodriguez, M.; Romero, I.; Llobet, A. Inorg. Chem.
003, 42, 8385−8394.
50) Rodriguez, M.; Romero, I.; Llobet, A.; Deronzier, A.; Biner, M.;
(
(
1
(
4
(
1
(
2
(
Parella, T.; Stoeckli-Evans, H. Inorg. Chem. 2001, 40, 4150−4156.
(
(
51) Roecker, L.; Meyer, T. J. J. Am. Chem. Soc. 1987, 109, 746−754.
52) Standard Potentials in Aqueous Solution, 1st ed.; Bard, A. J.,
Parsons, R., Jordan, J., Eds.; Marcel Dekker: New York, 1985.
53) Ozawa, H.; Hino, T.; Ohtsu, H.; Wada, T.; Tanaka, K. Inorg.
Chim. Acta 2011, 366, 298−302.
54) Serra, D.; Correia, M. C.; McElwee-White, L. Organometallics
011, 30, 5568−5577.
55) Paul, A.; Hull, J. F.; Norris, M. R.; Chen, Z.; Ess, D. H.;
Concepcion, J. J.; Meyer, T. J. Inorg. Chem. 2011, 50, 1167−1169.
56) Hembre, R. T.; McQueen, J. S.; Day, V. W. J. Am. Chem. Soc.
996, 118, 798−803.
57) Romming, C.; Smith, K.-T.; Tilset, M. Inorg. Chim. Acta 1997,
59, 281−290.
58) Wu, X.; Liu, J.; Di Tommaso, D.; Iggo, J. A.; Catlow, C. R. A.;
Bacsa, J.; Xiao, J. Chem. Eur. J. 2008, 14, 7699−7715.
59) Muller, J. G.; Acquaye, J. H.; Takeuchi, K. J. Inorg. Chem. 1992,
1, 4552−4557.
60) Sala, X.; Poater, A.; Romero, I.; Rodrıg
Solans, X.; Parella, T.; Santos, T. M. Eur. J. Inorg. Chem. 2004, 2004,
12−618.
61) Thai, T.-T.; Therrien, B.; Su
011, 696, 3285−3291.
(
(
2
(
(
1
(
2
(
(
3
(
́
uez, M.; Llobet, A.;
6
(
̈
ss-Fink, G. J. Organomet. Chem.
2
(
62) Singh, P.; Singh, A. K. Organometallics 2010, 29, 6433−6442.
(
63) Huynh, M. H. V.; Meyer, T. J. Chem. Rev. 2007, 107, 5004−
5
064.
(
(
64) Moyer, B. A.; Meyer, T. J. Inorg. Chem. 1981, 20, 436−444.
65) Cooks, R. G.; Ouyang, Z.; Takats, Z.; Wiseman, J. M. Science
2
(
4
(
2
(
006, 311, 1566−1570.
66) Takats, Z.; Wiseman, J. M.; Cooks, R. G. J. Mass Spectrom. 2005,
0, 1261−1275.
67) Takats, Z. W.; Wiseman, J. M.; Gologan, B.; Cooks, R. G. Science
004, 306, 471−473.
68) Perry, R. H.; Splendore, M.; Chien, A.; Davis, N. K.; Zare, R. N.
Angew. Chem., Int. Ed. 2011, 50, 250−254.
69) Liu, P.; Lanekoff, I. T.; Laskin, J.; Dewald, H. D.; Chen, H. Anal.
Chem. 2012, 84, 5737−5743.
70) Ions corresponding to the cationic dimer (m/z 576.9340) and
(
(
+
dimer [2+2] (m/z 838.1272) are likely formed in the secondary
microdroplets after 2 is extracted from the surface, consistent with the
31
conclusions of our recent communication.
71) Several additional ions were observed that could not be readily
assigned.
(
1
4305
dx.doi.org/10.1021/ja4055564 | J. Am. Chem. Soc. 2013, 135, 14299−14305